coetaneous \koh-i-TEY-nee-uhs, adjective:

Of the same age or duration.

Bear with these distractions, with this coetaneous growth of the parts: they will one day be members, and obey one will.
— Ralph Waldo Emerson, Self-Reliance and Other Essays

We could say that all living people are contemporaneous but not necessarily coetaneous; they live at different age levels.
— Harold C. Raley, A Watch Over Mortality

Coetaneous stems from the Latin roots co- meaning “with, together with,” ætat- meaning “age,” and the suffix -aneus (which is an adjectival suffix meaning “resembling”).